The invention relates to adjusting beam scanning velocity to improve sharpness in a raster scanning display such as a cathode ray tube (CRT) display.
The sharpness of a video display may be improved by varying the horizontal scanning rate of the beam in response to variations of the luminance component of the video signal. The luminance signal is differentiated and the differentiated luminance signal is used to generate a current for driving an auxiliary beam deflection element, for example, a scan velocity modulation (SVM) coil to modify the horizontal scanning rate so as to emphasize contrast between light and dark areas of the display. For example, at a transition from black to white in a given horizontal scan line, the beam scanning speed is increased approaching the transition, thus making the display relatively darker in the black area of the transition. Upon passing the transition into the white area, the beam speed is decreased such that the beam dwells relatively longer, making the display relatively brighter. The reverse occurs in passing from light to dark.
The SVM coil operates to add or subtract from the magnetic horizontal beam deflection field applied by the main horizontal deflection coils. The angle of beam deflection is a function of horizontal rate scanning current, generally a sawtooth current. The horizontal rate scanning current causes the beam to sweep across a horizontal raster line at a vertical position determined by a vertical rate sawtooth current, coupled to the vertical deflection coils.
The sawtooth scanning drive currents are adjusted to account for the fact that. the display screen is substantially flat rather than spherical. A given amount of angular beam deflection produces a smaller linear horizontal displacement of the beam at the center of the flat screen and a greater amount at the edges of the screen, because the screen is relatively farther from the source of the beam when scanning at the edges of the screen than at the center of the screen.
It may be desirable to display, for example, on-screen-display (OSD) characters on the screen of the CRT. The SVM current is, typically, optimized for non-OSD visual content. Therefore, when OSD character is displayed on the CRT screen, the SVM current could, disadvantageously, even degrade picture sharpness for OSD visual content of the picture. In one prior art, circuitry is provided for selectively disabling normal SVM circuit operation, during OSD operation.
It may be desirable to produce a waveform for the SVM current optimized for OSD display and a different waveform optimized for non-OSD display and to select dynamically the appropriate waveform. The selection may be changed on a region-by-region basis of the screen of the CRT, in accordance with the presence or absence of OSD visual content.
In carrying out an inventive feature, a signal indicative of the start and stop positions of OSD insertions in corresponding regions of the CRT screen is provided. The waveform used for generating the SVM current is dynamically selected, in accordance with the start and stop position indicative signal. In this way, optimization can be separately obtained for OSD and for non-OSD visual contents.
In carrying out a further inventive feature, a selection of the waveforms of the SVM control signal that is coupled to the modulator is done outside the video signal path. As explain before, the SVM control signal varies in accordance with the beam position. Thereby, advantageously, different SVM current waveforms are produced for OSD and for non-OSD visual content, respectively.
In an arrangement embodying an inventive feature, a video signal provides, selectively, a first type of visual content and a second type of visual content. When each of the types of visual content is provided, horizontal scanning occurs at a first horizontal deflection frequency. A first control signal has a first value, when the video signal provides the first type of visual content and a second value, when the video signal provides the second type of visual content. A waveform generator is responsive to the video signal for generating a correction signal coupled to a deflection field producing arrangement for varying a deflection field in accordance with the video signal to produce scan velocity modulated deflection of an electron beam. The waveform generator is responsive to the first control signal for generating a first waveform of the correction signal, when the first control signal is at the first value and a different, second waveform, when the first control signal is at the second value.